That's kinda hard to explain in words. It's a pretty situational move, but it *****.
There's a T-shaped tetrimino, the "T". It's normally purple colored on the tetrisfriends site. If you rotate the "T" at the right moment in certain situation, you can form lines. Requires a ton of practice to master in my opinion. The pay off is huge since, breaking one line using the t-spin sends your opponent two lines back. And breaking two lines using t-spin (T-spin double) sends your opponent four lines, that's the same reward for forming four lines (a tetris). It's really good, and all the high level players who have mastered it absolutely destroy me unless I play fast and perfect.
Here's an example of some T-spinning:
http://www.youtube.com/watch?v=SiK0OK8DP-Y
For me, T-spinning in tetris is almost as hard as Up-B canceling with Dr. Mario,
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.
And Mark, don't say anything about how easy it is for you to Up-B cancel, I just don't have it in me right now dang it,
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. Everybody can't be glitchmastaRenth ok.
EDIT:
WHOA, I just learned that you can break three lines using t-spin, a T-spin triple!! I've never seen it before now. I wonder how many lines that sends the opponent.
DOUBLE EDIT:
I just read a comment on that video that says T-spin triples send 6 attack lines to the opponent. That's awesome!